1. Field of the Invention
The present invention relates to a diesel particulate filter apparatus (hereinafter referred to simply as a DPF apparatus), which is incorporated in an exhaust system to clean exhaust gases discharged form diesel engines.
2. Description of the Prior Art
Combustion of diesel engine involves a so-called heterogeneous mixing whereby a fuel is injected into a high-temperature, high-pressure air. Unlike a homogeneous mixture, the heterogeneous mixture has its components, air and fuel, not uniformly mixed, so that carbon component in the fuel is heated during combustion to high temperature to become soot and hydrocarbons (AS) which coagulate into particulates and are discharged outside, contaminating the surrounding environment.
Conventional exhaust gas processing equipment that treat exhaust gases from engine have a filter with a large area to arrest carbon at its front part. To burn up the accumulated carbon, the particulate filter is provided with a heater that burns particulates including deposited carbon to regenerate the filter.
Japan Patent Laid-Open No. 256812/1990 discloses a particulate trap that can be regenerated electrically. The electrically regenerative particulate trap comprises a gas transmissive support, a fiber layer made up of a plurality of ceramics fibers, and a heater. The fiber layer is disposed together with the heater in a space radially extending from the gas transmissive support. The exhaust gas is passed through pores in the fiber layer to collect particulates contained in the exhaust gas into an area near the heater. By supplying electricity to the heater, the collected particulates are burned up. The fiber layer is a woven cloth with a uniform porosity of less than 1.5 L/M/cm.sup.2 and which remains stable for long period at about 1,150.degree. C. and has gaps between fibers not greater than 7.5 times the fiber diameter. The material of the ceramics fiber includes continuous molten silica, glass, alumina silica, zirconia silica, alumina chromia, titania, graphite, silicon carbide, and alumina boria silica. The ceramics fibers are braided or wound directly on the gas transmissive support or heater.
The DPF apparatus, because it is mounted on automobiles, is required to be formed as compact as possible and efficient in collecting particulates such as carbon, soot and HC. Particulates in general are synthesized substances from carbon and hydrocarbon and vary in size from several .mu.m to several tens of .mu.m. The particulates easily burn on reaction with oxygen but require high temperatures for combustion, and hence they cannot completely burn only with the exhaust gas heat. The conventional regenerative DPF apparatus has two filters of the same size parallelly arranged in the direction of exhaust gas flow. When one of the filters becomes clogged with arrested particulates, the exhaust gas is passed through the other filter, with the clogged filter supplied with air in an amount required for combustion of the accumulated particulates and then heated to burn the particulates to regenerate the filter.
There have been developed many regenerative DPF apparatuses that can remove particulates and smoke, but they have the drawback of being complex in construction and function and thus cannot be put into practical use. For instance, the particulates arrested by the filter, when they accumulate, are heated and burned by a spark glow plug or heater.